Search

International audience; Tree microhabitats (cavities, conks of fungi, bark features) play an important role for both rare and specialized species biodiversity; their preservation should therefore be targeted by biodiversity-friendly forest practices. However, when compared to other old-growth characteristics like deadwood or large trees, tree microhabitats have only recently been studied so related scientific knowledge is still relatively limited. Defining target values for microhabitat densities in managed forests is mostly based on expert knowledge rather than quantitative empirical data. We compared the densities of microhabitat-bearing trees between managed forests, where wood is still harvested, and strictly protected forest reserves, where harvesting has been abandoned, in 17 French forests (222 plots) located in both lowlands and mountain regions. We found that microhabitat densities are generally higher in strict forest reserves than in managed forests and that this difference is mainly driven by standing dead and large living trees. Though scarce, standing dead trees over-contribute to the difference observed while large trees played a lesser but significant role. In addition, contrary to results obtained for other old-growth characteristics (such as deadwood volumes), the difference between managed and strict forest reserves was higher in mountain than in lowland forests. For individual microhabitats, five out of eleven microhabitats in mountains and only one in lowland (woodpecker cavities) were significantly more numerous in strict forest reserves than in managed forests. Finally, total microhabitat density and density of specific microhabitats such as cavities and bark features increased with time since the last harvest. This increase was also mainly supported by standing dead microhabitat-bearing trees. Compared to previous studies in comparable contexts, the densities we estimated were generally higher; however, such comparisons could only be made for the most documented microhabitat types. Our results support the idea that management abandonment favours the abundance and diversity of microhabitats. However, microhabitat dynamics remain poorly known and only long-term monitoring will help understand underlying mechanisms of recruitment. In the meantime, our results may inspire forest managers in their application of daily biodiversity-friendly practices.

International audience; Riparian habitats are transitional zones where strong environmental gradients shape community. To prevent flood risks and channel migration on managed rivers, civil engineering techniques have been widely used. Recently, ecological restoration of rivers has become a major issue. However, given the alteration of natural disturbance regimes induced by human infrastructures, the real added-value of these restoration actions is questionable. Thus, a major challenge is to better understand whether changes in abiotic conditions induced by human activity influence the response of plant communities to environmental gradients. Studying a highly degraded large river, we evaluated the effect of the elevation and soil texture gradients on plant functional composition and assessed whether human-mediated environment gradients, achieved through maintenance and restoration measures, shape community structure. In the summer of 2017, we sampled 17 geomorphic surfaces, mostly gravel bars, along the Rhône River and its tributaries that were either repeatedly cleared (brush clearing vs plowing), newly reprofiled or naturally rejuvenated by high flows. The results show shifts in trait values with elevation and convergence in plant traits with increasing proportion of fine sediments. The co-occurrence of species with contrasting traits was higher in highly disturbed environments, revealing the importance of rejuvenation processes. However, the influence of both environmental gradients was mediated by human activity. For maintenance measures, plowing was better able to promote species diversity and limited biotic homogenization along environmental gradients. Among the three geomorphic surfaces, naturally rejuvenated bars were the most stressful environments, hosting distinct functional assemblages, while communities on newly reprofiled banks were in the same ecological trajectories as repeatedly cleared bars. To promote an effective ecological restoration of riparian zones, (i) a greater variability of the minimum flow is needed, (ii) bedload transport restoration should be a priority and (iii) reprofiled banks should better mimic the landforms of natural river margins.

International audience; Over the past centuries in Western Europe, the long history of forest management has shaped both landscape- and local-scale forest structures. In France, a network of strict forest reserves was created in the 1950’s to serve as a reference for nature conservation and forest dynamics. However, few studies to date have quantitatively compared the structural characteristics between managed stands and the reserves, where management has been (more or less recently) abandoned. In addition, the time needed for old-growth characteristics to restore themselves in strict forest reserves remains poorly known. We analysed the differences in stand characteristics between managed and unmanaged stands in 17 French forests (233 plots) located in both lowland and in mountainous regions. We quantitatively showed that overall basal area, very large trees and deadwood features were significantly higher in unmanaged forests, though this pattern was mainly found in the lowland forests while in mountainous regions, managed and unmanaged forests showed far fewer differences. In addition, most structural characteristics that we higher in unmanaged forests also increased with the time since last harvesting. Compared to the remaining old-growth forests elsewhere in Europe, the strict forest reserves in France often appear less mature, since they are still recovering from centuries of intensive management. Our results also show lower levels than those observed in other European studies on the restoration of old-growth characteristics over time; this indicates that it will take French reserves a very long time to reach values comparable to old-growth references in Europe. Our study constitutes one of the first references on unmanaged forests in France and may serve practitioners in their everyday management.

Extending the network of strict forest reserves is one of the conservation measures promoted by the French National Strategy for Biodiversity improvement. According to the scientific literature, strict forest reserves may help preserving a part of the biodiversity that is threatened by forestry. However, this strategy is based on poor knowledge in the temperate context and available studies may suffer from methodological shortcomings. The national-scale project named "Forest management, Naturalness and Biodiversity" aims at quantifying the effects on forest structure and biodiversity of management abandonment in strict reserves. Based on a meta-analysis of worldwide literature and original data from 213 study plots set up in 15 forest sites throughout France - around half of the plots in forests left unmanaged for from 8 to 148 years (mean: 46 years)- , we analyzed the response of 7 taxonomic groups to management abandonment. The meta-analysis demonstrates that forestry affects total richness of saproxylic taxa worldwide, which is verified in our national dataset. However, management abandonment per se is not always the best explanation of the differences between managed and unmanaged forests, but other variables, notably linked to deadwood, better explain the observed patterns for saproxylic groups. For other taxa, the response is weaker and depends more on structural features than on management abandonment. In terms of policy, our project has allowed methodological advances thanks to the development of inventory and remote sensing protocols, as well as statistical methods. The dataset we have gathered is also a first comparison of structure and biodiversity between strict forest reserves and managed forests for France. This network may therefore constitute a first basis for long term biodiversity monitoring in French forests.

International audience; Forest biodiversity assessment relies mostly on indirect indicators based on elements of forest structure used as a surrogate for species. However, most of those indirect indicators used in national and international evaluation processes lack scientific evidence documenting their quantitative link with the biodiversity they are supposed to assess. In addition, those for which the link is established generally concern only one taxon and are rarely considered at the community level. In this context, tree microhabitats – defined as tree-borne singularities such as cavities, conks of fungi or bark characteristics – have recently gained attention as a potential forest biodiversity indicator for species that depend on them for at least a part of their life cycle and that are difficult to detect. However, like most other potential indicators, precise quantitative information on the link between biodiversity indices and microhabitat variables remains scarce. We explored the link between the richness of three taxonomic groups (bats, birds, and saproxylic beetles) and microhabitat indices using a nation-wide multi-taxonomic sampling design comparing 213 plots located inside and outside forest reserves. We hypothesized that the positive effect of setting aside forest reserves on biodiversity conservation was indirectly due to the increase in the proportion of large structural elements (living trees and standing deadwood) which in turn favoured the quantity and diversity of microhabitats. We also hypothesized that microhabitat indices would be better indicators than large and standing dead tree densities, two classical and often used indirect forest biodiversity indicators. We first analysed the response of total species richness and species richness of different groups and guilds (e.g. red-listed species, forest specialists, cavity dwellers...) to several microhabitat indices (density, diversity) to identify the best microhabitat metric. We then used confirmatory structural equation models to assess the indirect effects of management, large structural elements and microhabitats on the diversity of the target species. We first showed that microhabitat indices based on diversity better described biodiversity than indices based on abundance. Structural equation models confirmed that setting aside forest reserves increased the quantities of large trees and standing deadwood, which in turn drove higher quantities and diversity of microhabitats. This significantly increased the richness of several taxonomic groups including notably total richness and cavity dweller richness of both birds and bats. However the magnitude of the link between microhabitat indices and species richness was relatively small. Conversely, the biodiversity of saproxylic beetles was not driven by microhabitat indices and, for this taxon only, models involving direct effects of large structural elements or management performed better. Tree microhabitats appear to be an indicator for part of the taxonomic groups we analysed, but do not constitute a universal biodiversity indicator. They rather have a complementary role compared to large structural elements. Currently, their moderate correlation with biodiversity, as well as thei sensivity to other factors not taken into account in this study (e.g. observer effects), question their transferability to management and policy. Finally, this study calls for replicable and standardized multicriteria methods to validate biodiversity indicators.

National audience; Nous présentons l'état de nos réflexions et les leçons tirées sur la base du projet GNB du point de vue du rôle respectif des gestionnaires et des chercheurs. C'est aussi l'occasion de présenter GNB et certains de ses résultats.

National audience; D'envergure nationale, l'objectif principal du projet "gestion forestière, naturalité et biodiversité (GNB)" est d'étudier l'impact de l'arrêt de l'exploitation forestière sur la structure des peuplements et sur la biodiversité.

SUMMARYSustainable forest management aims to produce wood while preserving habitats for biodiversity, which is particularly challenging for vertebrates with local and landscape scale requirements, such as birds or bats. Managers need additional scientific evidence to help them balance conservative and integrative management methods. In this study, the relative influence of management abandonment, stand structure and landscape features on bird and bat communities in 14 managed and unmanaged forests in France is evaluated. Total birds and bats richness, richness for forest and threatened birds and edge-specialized bats significantly increased with total deadwood quantities. Richness of generalist, omnivorous and cavity-nesting birds was higher in unmanaged stands and richness of gleaner bats was positively influenced by the density of standing deadwood. By contrast, landscape variables had little influence on the different ecological groups but did have effects on individual species. Though the effects showed relatively limited magnitude, this study supports the value of deadwood and the importance of management abandonment for forest vertebrates. This study confirms that for integrated conservation strategies to be fully efficient they must be complemented by designating strict forest reserves because some target species groups depend on structural features found only at sufficient levels in those areas.

Extending the network of strict forest reserves is one of the conservation measures promoted by the French National Strategy for Biodiversity improvement. According to the scientific literature, strict forest reserves may help preserving a part of the biodiversity that is threatened by forest management. However, this management choice is based on poor knowledge in the French context and the studies concerned may suffer from methodological shortcomings. The national-scale project named “Forest management, Naturalness and Biodiversity” aims at quantifying the effects on forest structure and biodiversity of management abandonment in the strict reserves. Based on a worldwide meta-analysis and 213 study plots set up in 15 forest sites throughout France, we analysed the response of 7 taxonomic groups to management abandonment. On the one hand, we show that forest management affects total richness of saproxylic taxa worldwide, in particular bryophytes and saproxylic fungi. On the other hand, this trend is verified on our dataset. However, management abandonment per se is not always the best explanation of the differences between managed and unmanaged forests, but other variables, notably linked to deadwood, better explain the observed patterns for these groups. For the other taxa, the response is weakest but depends more on structural features than on management abandonment. In terms of policy, our project has allowed methodological advances thanks to the development of inventory and remote sensing protocols, as well as statistical methods. The dataset we have gathered is also a first comparison of structure and biodiversity between strict forest reserves and managed forest for France. This network may therefore constitute a first basis for long term biodiversity monitoring in French forests., L'extension raisonnée du réseau de réserves forestières intégrales fait partie des mesures adoptées par la Stratégie Nationale pour la Biodiversité pour améliorer la biodiversité forestière métropolitaine française. Au regard de la littérature scientifique, la non-exploitation favoriserait une partie de la biodiversité forestière menacée par la gestion forestière traditionnelle, mais les connaissances qui sous-tendent ce choix se révèlent très partielles et souffrent de problèmes méthodologiques. D’envergure nationale, le projet « Gestion forestière, Naturalité et Biodiversité » (GNB ; http://gnb.irstea.fr) a pour objectif principal d’étudier l’impact de l’arrêt d’exploitation forestière dans le réseau des réserves forestières intégrales sur la structure des peuplements et sur la biodiversité Sur la base d’une méta-analyse et de 213 placettes installées dans 15 massifs forestiers français, nous avons analysé la réponse de 7 groupes taxonomiques à l’arrêt d’exploitation. Nous montrons d’une part que les résultats mondiaux confirment que l’exploitation affecte la richesse des taxons saproxyliques, notamment bryophytes et champignons, et que, d’autre part, cette tendance se vérifie pour le jeu de données issu du projet. Cependant, la mise en réserve en tant que telle n’est la plupart du temps pas la meilleure explication des différences entre peuplements exploités et non exploités, mais d’autres variables, liées au bois mort notamment, expliquent mieux les patrons pour ces groupes. Pour les autres taxons, la réponse est plus faible mais dépend plus de la structure du peuplement que de la mise en réserve. En termes d’appui aux politiques publiques, le projet a permis des avancées méthodologiques notamment grâce au développement de protocoles d’inventaires et de télédétection, et de méthodes statistiques. Le jeu de données acquis constitue par ailleurs un premier état des lieux de la structure forestière et de la biodiversité des réserves forestières intégrales en France, en comparaison avec des forêts exploitées. Ce dispositif pourrait ainsi servir de première base à un suivi au long cours de la biodiversité des forêts françaises.

International audience; The decline of many saproxylic species results from the decrease in old-growth structures in European harvested forests. Among conservation tools, protected reserves withdrawn from regular harvesting and extended rotations have been employed to restore old-growth attributes in structurally simplified managed forests, even if the effects of such management actions on forest habitats and biodiversity remain largely unknown. In this study, we compared structural stand features and saproxylic beetle assemblages in two stand classes - recently harvested stands and long-established reserves, where less or more than thirty years had elapsed since last harvest. Habitat and saproxylic beetle data were collected according to standardized protocols in 153 plots in seven lowland deciduous forests. Tangible contrasts in stand features were found between long-established reserves and recently-harvested plots. Indeed, most higher-value densities and volumes were found in unharvested areas. The difference was weaker for microhabitat-bearing tree density than for deadwood; some deadwood features, such as volume of large downed and standing deadwood showed a very pronounced difference, thus indicating a marked deleterious effect of forest harvesting on these elements. Deadwood diversity, on the other hand, was only slightly affected and the level of stand openness did not change. The response of saproxylic beetles to delayed harvesting was weaker than the structural changes in deadwood features. Indeed even if only some guilds weakly increased in non-harvested plots, harvesting classes significantly affected the abundance of a quarter of the species tested. Our results tend to question measures such as rotating and temporarily ageing patches. We argue in favor of permanent strict fixed-location reserves. Future work should examine how stands recover old-growth forest attributes and how the associated saproxylic fauna colonises in the long-term.

National audience; Nous présentons ici un bilan des premiers résultats du projet GNB. Nous insistons notamment sur les variables écologiques liées au peuplement arboré, à la durée d'abandon et au spatial. Cette présentation est par ailleurs l'occasion de remercier les personnes ayant contribué à GNB.

National audience; Le réseau des réserves forestières (naturelles et biologiques) constitue un formidable support d’étude de la dynamique et de la biodiversité forestière. Néanmoins, et jusque récemment, seuls quelques sites avaient fait l’objet d’une évaluation précise du bois mort, dont l’importance pour la survie de nombreuses espèces forestières n’est plus à démontrer. Dans ce contexte et à la demande des gestionnaires, un protocole de suivi dendrométrique destiné aux réserves forestières (PSDRF) a été élaboré en 2005, sous la conduite de l’ENGREF. Par le biais de placettes permanentes, sa mise en oe½uvre permet non seulement de décrire les peuplements forestiers échantillonnés mais aussi d’approfondir les connaissances sur la dynamique de ces peuplements dans le temps et l’espace. Coordonné depuis 2008 dans le cadre d’un partenariat entre l’ONF et RNF, ce protocole a, à ce jour, été appliqué sur plus de 80 réserves, soit plus de 7000 placettes relevées sur près de 40 000 ha et couvrant ainsi la majorité des grands types d'habitats forestiers français. En moyenne, on observe que les réserves contiennent un volume de bois mort non négligeable (moyenne : 37,1 m3/ha, variant de 5 à 150 m3/ha). A quelques exceptions près, les réserves intégrales contiennent plus de bois mort que les réserves qui font l’objet d’une exploitation.

National audience; Le but de cette étude est d’évaluer l’influence respective de l’exploitation forestière, de la structure et du paysage sur les communautés de chiroptères et d’oiseaux en forêt exploitée et non exploitée en France. Nos résultats montrent que la richesse totale de l'avifaune et des chiroptères, la richesse des oiseaux forestiers et des chiroptères de lisière augmente faiblement avec le volume total de bois mort. La richesse des oiseaux généralistes, des omnivores et des cavicoles est légèrement plus élevée dans les forêts non exploitées. Les chiroptères glaneurs sont positivement influencés par la densité de bois mort debout. Par contre, les variables paysagères retenues n’ont que peu d’influence sur la biodiversité. Malgré des effets assez faibles, notre étude confirme l’intérêt du bois mort pour la diversité de ces groupes.

International audience; In Western Europe, the long history of forest management over the past centuries has shaped both landscape and local scale forest structure, presumably altering the biodiversity of forest dwelling species. In France, the strict forest reserves network has been created to serve as a witness to gauge the effects of management on forest structures and dynamics: it currently covers 0.3% of national territory, distributed over 200 sites representative of the main forest types. However, to date, research comparing biodiversity in managed and unmanaged forests remains strikingly poor in Western Europe, and hardly proposes a broad taxonomic assessment. In order to fill the gap in knowledge in the French context, we studied forest structure and biodiversity in lowland oak-beech-hornbeam forests, and in montane beech-fir-spruce forests. We compared living and dead wood amounts and biodiversity of 6 taxa (vascular plants, saproxylic fungi, birds, bats, carabids and saproxylic beetles) 15 strict forest reserves where forest management has been abandoned for at least 20 years and adjacent managed forests, totalizing 213 plots. Relatively to managed forest, stands in unmanaged forest host larger amounts of old-growth components (deadwood volumes, number of large trees) as well as higher basal areas, whereas other stand characteristics did not differ significantly. Saproxylic fungi responded significantly to management abandonment, their total richness increasing with time since last harvesting. Responses of the other taxonomic groups were less clear and further analyses correlating forest structure with taxa or ecological groups are necessary to better understand the mechanisms associating biodiversity and forest management. The applied outcome of this multitaxonomic approach is to validate indirect biodiversity indicators on a broad gradient of forest management. This project constitutes the first reference for mixed beech forests in France and may help managers to define thresholds in terms of biodiversity oriented measures.

National audience; Au carrefour entre différentes régions biogéographiques, la France héberge une grande diversité d'habitats et de peuplements forestiers. Cette diversité résulte non seulement de la combinaison de facteurs géographiques, géologiques et climatiques, mais également de siècles d’exploitation qui ont façonné le paysage forestier. Supports de la présente étude, les réserves biologiques (gérées par l’ONF) et les réserves naturelles (regroupées au sein de RNF), à forte composante forestière, sont représentatives de cette diversité des forêts françaises. Elles présentent également un gradient de gestion important, allant de l’exploitation mécanique à la libre évolution. Jusqu'à récemment, les estimations du volume de bois morts dans les forêts françaises étaient extrêmement imprécises. En cause : (1°) une prise en compte inadaptée dans les méthodes courantes d'inventaires, tournées vers le suivi de la ressource et (2°) un très faible nombre de sites étudiés de façon précise (e.g. Fontainebleau, Massane). Dans ce contexte, à la demande des gestionnaires et sous l'égide du ministère de l'Ecologie, le protocole de suivi dendrométrique des réserves forestières (PSDRF) a été élaboré en 2005. Coordonné depuis 2008 dans le cadre d’un partenariat entre l’ONF et RNF, ce protocole a, à ce jour, été appliqué sur plus de 80 réserves, représentant près de 7000 placettes et couvrant ainsi la majorité des grands types d'habitats forestiers français. Par le biais de placettes permanentes, sa mise en oeuvre permet non seulement de décrire les peuplements forestiers échantillonnés (et donc de participer à l’évaluation de l'état de conservation des habitats) mais aussi, et c’est la principale originalité de ce protocole, d’approfondir les connaissances sur la dynamique de ces peuplements dans le temps et l’espace (accroissement naturel, vitesse de décomposition, flux de nécromasse, etc.). Basés sur des données dendrométriques collectées dans quelques 60 réserves, les premiers résultats montrent que la plupart des réserves forestières contiennent un volume de bois mort nonnégligeable (moyenne : 37,1 m3/ha, fourchette allant de 5 à 150 m3/ha). A quelques exceptions près, les réserves intégrales (soustraites à toute exploitation) contiennent plus de bois mort que les réserves qui font l’objet d’une exploitation. Comparé aux résultats préliminaires du dernier inventaire IGN (2008-2010), les réserves forestières se trouvent à des niveaux de bois mort globalement plus élevés que pour la forêt française dans son ensemble (23,1 m3/ha). C’est particulièrement le cas pour les arbres morts sur pied mais certaines différences qualitatives sont également identifiées pour le bois mort au sol. Par ailleurs, dans sa phase initiale de description, des inventaires taxonomiques peuvent venir se superposer au PSDRF. Ce type de recherche-action, impliquant gestionnaires et chercheurs, permet de valoriser des données dendrométriques en leur associant des données de biodiversité. Initié en 2008 sous l'impulsion d'Irstea, de l'ONF et de RNF, le projet "Gestion forestière, Naturalité et Biodiversité" (GNB) vise ainsi à mieux appréhender les effets de l’arrêt de l’exploitation forestière sur la biodiversité de 7 groupes taxonomiques (plantes vasculaires, bryophytes, champignons lignicoles, oiseaux, chauve-souris, carabes et coléoptères saproxyliques) en comparant des forêts couramment exploitées et des réserves où l'exploitation a été stoppée depuis au moins 20 ans. A ce jour, 213 placettes ont été échantillonnées sur 15 massifs (gnb.irstea.fr). Restreint aux habitats de hêtraies-chênaies en plaine et de hêtraies-sapinières de montagne, les résultats préliminaires du projet GNB montrent les mêmes tendances pour le bois mort. En effet, les volumes de bois mort sont les caractéristiques qui diffèrent le plus entre zones exploitées et non exploitées, avec en moyenne 4,5 fois plus de bois mort en zone non exploitée. En termes de biodiversité, les premiers résultats montrent que la mise en réserve a une influence différente en fonction du taxon étudié. Un approfondissement des analyses devrait mettre en évidence les meilleurs facteurs structurant la biodiversité forestière.

/ Le but de cette étude est d'évaluer l'influence respective de l'exploitation forestière, de la structure et du paysage sur les communautés de chiroptères et d'oiseaux en forêt exploitée et non exploitée en France. Nos résultats montrent que la richesse totale de l'avifaune et des chiroptères, la richesse des oiseaux forestiers et des chiroptères de lisière augmente faiblement avec le volume total de bois mort. La richesse des oiseaux généralistes, des omnivores et des cavicoles est légèrement plus élevée dans les forêts non exploitées. Les chiroptères glaneurs sont positivement influencés par la densité de bois mort debout. Par contre, les variables paysagères retenues n'ont que peu d'influence sur la biodiversité. Malgré des effets assez faibles, notre étude confirme l'intérêt du bois mort pour la diversité de ces groupes.

National audience; Nous avons étudié l’impact de la durée de non-exploitation de parcelles forestières sur les caractéristiques du milieu (en particulier variables d’habitat favorables aux saproxyliques), ainsi que sur les assemblages en coléoptères saproxyliques. Après 30 ans de non exploitation, les éléments caractéristiques du milieu favorables aux saproxyliques (bois mort, micro-habitats…) ont significativement augmentés (volume et nombre). Pourtant, la richesse et l’abondance des espèces (rares en particulier) n’est que peu impactée. Nous émettons différentes hypothèses pour expliquer ce phénomène.

/ Le réseau des réserves forestières (naturelles et biologiques) constitue un formidable support d'étude de la dynamique et de la biodiversité forestière. Néanmoins, et jusque récemment, seuls quelques sites avaient fait l'objet d'une évaluation précise du bois mort, dont l'importance pour la survie de nombreuses espèces forestières n'est plus à démontrer. Dans ce contexte et à la demande des gestionnaires, un protocole de suivi dendrométrique destiné aux réserves forestières (PSDRF) a été élaboré en 2005, sous la conduite de l'ENGREF. Par le biais de placettes permanentes, sa mise en oe½uvre permet non seulement de décrire les peuplements forestiers échantillonnés mais aussi d'approfondir les connaissances sur la dynamique de ces peuplements dans le temps et l'espace. Coordonné depuis 2008 dans le cadre d'un partenariat entre l'ONF et RNF, ce protocole a, à ce jour, été appliqué sur plus de 80 réserves, soit plus de 7000 placettes relevées sur près de 40 000 ha et couvrant ainsi la majorité des grands types d'habitats forestiers français. En moyenne, on observe que les réserves contiennent un volume de bois mort non négligeable (moyenne : 37,1 m3/ha, variant de 5 à 150 m3/ha). A quelques exceptions près, les réserves intégrales contiennent plus de bois mort que les réserves qui font l'objet d'une exploitation.

International audience; Deadwood, whether expressed in volume, diversity or continuity, is nowadays widely acknowledged as being an important component of forest ecosystems, and has therefore been addressed in most recent studies dealing with forest conservation and management. However, and despite the growing use of the concept of naturalness in the management plans of French forest reserves, the dynamics of deadwood remains greatly unknown in West European temperate forests. In this context, and in order to answer a growing demand arising from the managers of forest reserves, a specific long-term monitoring scheme has been implemented in all main forest habitats since its launch in 2005. Our talk will present some preliminary results of this country-wide monitoring based on data from 36 Biological Reserves and 18 National Nature Reserves. In this network, a comprehensive data set of living and dead tree measurements has already been collected on >5.000 permanent plots covering a wide range of situations. Although most of the sites showed significant amounts of deadwood (mean: 38.1 m3/ha), we will focus our presentation on the important differences that were found between sites and within sites. We analyze the relationships between these differences and habitat types, altitude and management practices. Future surveys will allow us to better understand the driving forces behind these differences and will provide us with management tools to assess how these values will/can increase in the future.

International audience; Deadwood, whether expressed in volume, diversity or continuity, is nowadays widely acknowledged as being an important component of forest ecosystems and has therefore been addressed in most recent studies dealing with forest conservation and management. However, and despite the growing use of the concept of naturalness in the management plans of French forest reserves, deadwood dynamics remain greatly unknown in European temperate forests. In this context and to answer a growing demand arising from reserve managers, a long-term monitoring scheme has been launched in 2005. Preliminary results from 42 French forest reserves will be presented in this talk. A comprehensive data set of living and dead tree measurements have so far been collected on more than 4000 permanent plots. Sampling is systematic but the density of plots is adjusted to the reserve size, habitat diversity and management practices. Overall, French forest reserves host an average of 35.1 m3/ha of deadwood. More than 2/3 of the reserves had deadwood volumes between 20 and 40 m3/ha, and were considered deadwood rich. For more than 1/3 of these reserves, the estimated volume was even greater than 40 m3/ha, indicative of forests growing under nearly natural conditions. Although most of the reserves showed a significant amount of deadwood, the gap with old-growth forests is still of great magnitude. The following surveys should allow us to better understand these differences.

National audience; Une étude du Cemagref livre ses premiers résultats; suprenants: la fréquence des micro-habitats dépend moins du mode de gestion que des caractéristiques des arbres.

International audience; L'exploitation forestière est susceptible de modifier profondément la biodiversité. La forte diversité des forêts tempérées est menacée au même titre que la diversité d'autres milieux. Cependant, la réponse de la biodiversité à l'exploitation forestière n'est que partiellement connue. L'objectif de ce projet est de quantifier la réponse de la biodiversité de sept groupes taxinomiques : bryophytes, champignons, plantes vasculaires, coléoptères saproxyliques et carabiques, oiseaux et chauves-souris à l'exploitation forestière et à diverses facteurs de gestion et de naturalité, en comparant dans les mêmes massifs des zones exploitées et non exploitées (Réserves Biologiques Intégrales et Réserves Naturelles), en plaine et en montagne. L'objectif en 2008 est de tester la faisabilité du projet sur 3 massifs. A terme, nous espérons échantillonner plus de 20 massifs.

National audience; Based on a comparison between managed and unmanaged forests, the aim of our study is to provide managers with a tentative quantitative analysis of the structural differences between managed and unmanaged mixed beech forests in France from a sample of 213 plots located in 15 lowland and mountain forests. Most structural descriptors were significantly higher in unmanaged forests. There were more very large living trees (+ 105 %) in unmanaged than in managed forests. These trees were also bigger (+ 145 %). The volume of dead wood was four times higher in unmanaged forests. Because of its nationwide scope, our investigation is a first French benchmark study for biodiversity-oriented forest policy, and contributes additional knowledge to a poorly researched area in France.; Basée sur la comparaison de forêts exploitées et non exploitées, l’objectif de notre étude est de fournir aux gestionnaires forestiers une première analyse quantitative sur les différences structurelles des hêtraies mélangées exploitées et non exploitées de France, sur un dispositif d’échantillonnage de 213 placettes réparties sur 15 massifs de plaine et de montagne. La majorité des descripteurs de structure est significativement plus élevée en forêts non exploitées. Les très gros bois vivants sont plus nombreux (+ 105 %) et plus gros (+ 145 %) en forêts non exploitées qu’en forêts exploitées. Le volume de bois mort est quatre fois plus grand en forêts non exploitées. De par son ampleur, notre étude constitue une première référence française en soutien aux politiques de conservation de la biodiversité, et apporte une pierre supplémentaire au champ de connaissance jusqu’à présent lacunaire en France.